An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include printheads with orifice plates with several very small nozzles through which the ink droplets are ejected. Adjacent to the nozzles are ink chambers, where ink is stored prior to ejection through the nozzle. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir part of the pen.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The thermal process causes ink within the chamber to superheat and form a vapor bubble. Formation of thermal ink-jet vapor bubbles is known as nucleation. The rapid expansion of ink vapor forces a drop of ink through the nozzle. This process is called "firing." The ink in the chamber may be heated with a resistor that is aligned adjacent to the nozzle.
Another mechanism for ejecting ink may employ a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of the ink in the firing chamber thereby to produce a pressure wave that forces the ink droplets through the printhead nozzle.
Previous ink-jet printheads rely on capillary forces to draw ink through an ink channel and into an ink chamber, from where the ink is ejected. Once the ink is ejected, the ink chamber is refilled by capillary force with ink from the ink channel, thus readying the system for firing another droplet.
As ink rushes in to refill an empty chamber, the inertia of the moving ink causes some of the ink to bulge out of the nozzle. Because ink within the pen is generally kept at a slightly positive back pressure (that is, a pressure slightly lower than ambient), the bulging portion of the ink immediately recoils back into the ink chamber. This reciprocating motion diminishes over a few cycles and eventually stops or damps out.
If a droplet is fired when the ink is bulging out the nozzle, the ejected droplet will be dumbbell shaped and slow moving. Conversely, if the ink is ejected when ink is recoiling from the nozzle, the ejected droplet will be spear shaped and move undesirably fast. Between these two extremes, as the chamber ink motion damps out, well-formed drops are produced for optimum print quality. Thus, print speed (that is, the rate at which droplets are ejected) must be sufficiently slow to allow the motion of the chamber to damp out between each droplet firing. The time period required for the ink motion to damp sufficiently may be referred to as the damping interval.
To lessen the print speed reduction attributable to the damping interval, ink chamber geometry has been manipulated. The chambers are constricted in a way that reduces the ink chamber refill speed in an effort to rapidly damp the bulging refilling ink front. Generally, chamber length and area are constructed to lessen the reciprocating motion of chamber refill ink (hence, lessen the damping interval). However, printheads have been unable to eliminate the damping interval. Thus, print speed must accommodate the damping interval, or print and image quality suffer.
Ink-jet printheads are also susceptible to ink "blowback" during droplet ejection. Blowback results when some ink in the chamber is forced back into the adjacent part of the channel upon firing. Blowback occurs because the chamber is in constant fluid communication with the channel, hence, upon firing, a large portion of ink within the chamber is not ejected from the printhead, but rather is blown back into the channel. Blowback increases the amount of energy necessary for ejection of droplets from the chamber ("turn on energy" or TOE) because only a portion of the entire volume of ink in the chamber is actually ejected. Moreover, a higher TOE results in excessive printhead heating. Excessive printhead heating generates bubbles from air dissolved in the ink and causes prenucleation of the ink vapor bubble. Air bubbles within the ink and prenucleation of the vapor droplet result in a poor ink droplet formation and thus, poor print quality.
The present invention provides an assembly that includes minute, active valve members operable for controlling ink flow within an ink-jet printhead. An embodiment of the valve assembly is incorporated in an ink channel that delivers ink to the firing chambers of the printhead. The valve members include a resiliently deformable flap connected at one end to a surface of the ink channel. The free end of the flap is deflected into a position that restricts ink flow within the channel. The flap substantially isolates the ink chamber from the channel during firing of a droplet.
Isolating the chamber with the flap reduces blowback. During ejection, ink in the chamber is blocked by the deflected flap and cannot blowback into the channel, but must exit through the nozzle. This blowback resistance raises the system thermal efficiency, lowering TOE. A lower TOE reduces printhead heating. Reducing printhead heating helps maintain a steady operating temperature, which provides uniform print quality.
With the flaps deflected in a manner such that the ink chamber is isolated immediately after chamber refill, the valve assembly of the present invention also reduces the ink damping interval. With the chamber isolated, the distance the ink may travel back from the nozzle is limited, which in turn reduces the reciprocating motion of the ink. Consequently, the ink damping interval is significantly decreased, allowing higher print quality at faster printing speeds.